The present invention relates generally to the removal of sulfur oxides and other contaminants contained in a flue gas by spray drying or dry scrubbing. More particularly, it relates to an improved dry scrubbing apparatus which permits operation at or near the saturation temperature of the flue gas without deposition of the scrubbing particulates on the walls of the scrubbing apparatus.
The reduction of sulfur oxides and other contaminants contained in a flue gas formed during the combustion of coal/sulfur-bearing fossil fuels and waste materials is of the utmost concern to industry. These fuels are burned by electric power generating plants, waste recycling plants, and other industrial processes. There have been many attempts to comply with federal and state air pollution requirements which have included the treatment of the fuel to minimize sulfur as well as the treatment of the fuel combustion products to eliminate sulfur from the flue exhaust.
The obvious approach is to locate and utilize fossil fuels lower in sulfur content and/or other contaminants. The major disadvantage with this approach is the increased fuel and freight cost due to supply and demand and/or proximity to the end user of this fuel.
Another attempt in the reduction of the sulfur content in the fuel prior to combustion is to utilize mechanical and/or chemical processes which eliminate sulfur from the fuel. The major disadvantage with this approach is the cost effectiveness of the mechanical and/or chemical processing necessary to meet the required reduced levels of sulfur and/or other contaminants in the fuel.
The more acceptable approach has been to treat flue gas combustion products to remove sulfur while burning fuel that is high in sulfur content.
In one approach, dry pulverized alkali is injected directly into the hot combustion gases to remove sulfur oxides and other contaminants by way of chemical adsorption or absorption followed by oxidation. The major disadvantages with this approach is the low to moderate removal efficiencies, poor reagent utilization, and increased particulate loadings in the flue gas which necessitates the requirement for further flue gas conditioning (i.e., humidification) when the injection process is conducted upstream of a collection device such as an electrostatic precipitator (ESP).
Dry scrubbing is preferable to the above approach. In dry scrubbing, an aqueous alkaline solution or slurry is atomized via a mechanical, dual fluid, or rotary type atomizer and sprayed into hot flue gases downstream of particulate removal devices such as baghouses or electrostatic precipitators to remove sulfur oxides and other contaminants. The major disadvantage with this approach to date has been the limitation on spray down temperature. As the dry scrubbing apparatus approaches flue gas saturation temperature, the build up of particulates from the dried alkaline slurry on the walls of the dry scrubbing apparatus becomes severe.
One example of a known dry scrubber is shown in U.S. Pat. No. 5,194,076 to Myers et al., titled "Low Pressure Drop Dry Scrubber", assigned to The Babcock & Wilcox Company. Here the flue gas is treated by dry scrubbing with an aqueous alkaline solution or slurry which is finely atomized and sprayed into a hot flue gas. The atomized slurry droplets dry within the dry scrubber reactor and remove sulfur oxides. The resulting product is a low moisture, fine particulate that exits the dry scrubber via an outlet and is collected by a particulate collection device such as a baghouse, electrostatic precipitator, or other known means. This dry scrubber works in a commendable fashion for operation above flue gas saturation temperatures. However, when these saturation temperatures are approached the previously mentioned particulate deposition begins to occur on the reactor walls. Removal of this particulate build-up is a costly process, and requires the reactor to be shut down. Shut down requires either spare reactor modules, boiler and power generation shut down, or emissions which exceed regulations--alternatives that are unacceptable.
Another known dry scrubber is shown in U.S. Pat. No. 5,354,364 to Johnson et al., titled "High Efficiency Advanced Dry Scrubber", also assigned to The Babcock & Wilcox Company. Here, the apparatus and method for spraying a liquid into a flue gas provides a boundary layer of gas along a perimeter of a dry scrubber housing to prevent deposition of wet material on the sidewalls thereof. The boundary layer of gas includes the use of heated air, particulate free flue gas, and/or dust laden flue gas. As shown in FIGS. 4A and 4B thereof, and discussed at col. 4, lines 1-50, the boundary layer is preferably established by a plenum which is connected to a plurality of openings to provide the boundary layer gas flow on the inside surface and allow it to flow parallel with the sidewalls of the dry scrubber and prevent deposition thereon. As shown in FIG. 4A, the flue gas flow and the boundary layer flow are in the same direction. As shown in FIG. 4B, one method for mixing the heated boundary layer flow with the main flue gas stream, trip plates which may be either solid or perforated are provided to direct the boundary layer flow into the main flue gas stream.
The slurry and compressed air mixture exiting from the atomizers forms high velocity jets that have more momentum than the incoming flue gas. Since the atomizer mixture momentum is greater than the dry scrubber inlet flue gas momentum, it entrains the flue gas flow. This entrainment is desirable since it is the mechanism which intimately mixes the atomized slurry and flue gas flows together.
However, when the entrainment causes the flue gas to mix with the slurry jet, it reduces the jet velocity. If the jet is confined by the reactor walls, the jet may become starved and an insufficient amount of the flue gas will be available for entrainment. When this occurs, the jet attaches to the inside reactor walls and a recirculation zone or region forms in the flow field. This recirculation region recycles a mixture of the slurry (now partially dried) and the flue gas back towards the jet for re-entrainment. If the recirculating gas originates at a lower section of the dry scrubber reactor, it will contain mostly dry particulate which is unlikely to deposit on the reactor walls if the flow patterns remain stable. However, as efforts are made to improve dry scrubbing efficiencies (for example, operating when the outlet flue gas is at/or near saturation, and/or when the reactor walls are closer spaced, such as to reduce costs), the attachment point moves upward along the reactor walls and the particulate moisture content increases. This increases the probability that particulates will deposit on the reactor walls. In some circumstances, the recirculation pattern can become very small, and such flow patterns are unacceptable because they lead to an excessive amount of deposition on the reactor walls.
It is thus clear that preventing and avoiding particulate deposition is complicated by the flue gas and slurry flow patterns within the dry scrubber. A dry scrubber design that reduces or eliminates the above-identified recirculation region would be welcomed by the industry, since it would facilitate operation at temperatures at/or near saturation without excessive slurry particulate deposition on the reactor walls.